Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as medical, automobile, and other applications. The device architecture of image sensors has continued to advance at a great pace due to increasing demands for higher resolution, lower power consumption, increased dynamic range, etc. These demands have also encouraged the further miniaturization and integration of image sensors into these devices.
The typical image sensor operates as follows. Image light from an external scene is incident on the image sensor. The image sensor includes a plurality of photosensitive elements such that each photosensitive element absorbs a portion of incident image light. Photosensitive elements included in the image sensor, such as photodiodes, each generate image charge upon absorption of the image light. The amount of image charge generated is proportional to the intensity of the image light. The generated image charge may be used to produce an image representing the external scene.
While there are a variety of ways to make image sensors, reducing the number of steps with fewer photo masks in semiconductor processing applications is always important. Since every fabrication step adds cost and time on the assembly line, new techniques to enhance image sensor throughput are needed.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.